Evaluation of the Spiral Plating System for the Routine Assessment of Indicator Microorganisms in Raw Ewe's Milk

Abstract

The numbers of members of different microbial groups in bulk raw ewe's milk used for cheesemaking (46 samples, taken on receipt at the dairy over 1 year) were assayed by the spiral plating system to determine the effectiveness of this method compared with that of widely accepted conventional methods in providing counts. The results indicated that for ewe's milk, the suitability of the spiral plating system depends to a great extent on the microbial group studied. Although “spiral” counts of mesophiles, psychrotrophs, coliforms, and Enterobacteriaceae could be considered equivalent to those obtained by conventional techniques (r ≥ 0.90; variance between replicate platings ≈ 0.005), the automated method was found not to be suitable for the assessment of other groups of indicator bacteria (thermodurics and enterococci). Counts of lactic acid bacteria and yeasts and molds were affected significantly (P < 0.05) by the plating method, although other statistical parameters were more favorable (r = 0.88 and r = 0.82, respectively; 95% confidence limits within 0.5 log units). Finally, counts of staphylococci, particularly on Baird-Parker medium, showed less variation and higher reproducibility with the spiral method. Nevertheless, for the routine microbiological analysis of ewe's milk, the spiral plating system, with its time-, effort-, and material-saving advantages, is preferred over the conventional method.